Regenerative fuel cells, and the methods by which they are able to store and deliver electricity, have been known for many years. They are electrochemical apparatus for energy storage and power delivery. In the power delivery phase, electrochemically active species are supplied to electrodes, where they react electrochemically to produce electrochemical power. In a storage phase, electrical power is used to regenerate the electrochemically active species, which may be stored.
Because the electrochemically active species can be stored separately from the electrode compartments and supplied when required, the generating capacity of this equipment can be quite large.
The electrochemical reactions take place on either side of an ion transport system (such as a membrane) with selective charge carriers either being transported or exchanged by the membrane.
The fundamental chemical process in these regenerative fuel cell (RFC) systems is characterised by a chemical equation where the action proceeds in one direction in the energy storage mode of the system and in the opposite direction during the power delivery mode by the system. This chemical process can be exemplified by the following redox chemical equation, wherein the term “redox” defines reactions in which a reduction and a complementary oxidation occur together.VIV(sol)+VIII(sol)VII(sol)+VV(sol)  Equation 1
However the implementation of these systems in practical applications has encountered major limitations, despite what appears to be a simple direct chemical process. Practical problems including the use of hazardous materials, poor efficiencies, system size, plugging and clogging of the flow of the electrolytes, gas formation and especially the cost of materials and the cost of equipment. These have prevented RFCs from being employed widely in industry.
Currently RFCs use cation (positively charged ions) exchange or cation transfer membranes to separate the half cells containing the two electrodes. These may be polymers with negatively charged groups (typically —CO3− or —SO3−) grafted onto the polymer and cationic species which balance the charge (H+, Na+ etc). Therefore, many RFCs use acidic electrolytes, as this provides an abundance of cations to be exchanged/transferred, and allows the use of many well known redox couples which are highly soluble in acidic media. For example, EP0664932B1 describes an energy generation and power storage system using a bromine/bromide or Fe2+/Fe3+ reaction in one half cell and a sulphide/polysulphide reaction in the other half cell. A cation exchange membrane separates the two half cells, and during the power delivery and the energy storage modes, the charge balance for the chemical reaction is provided by the transport of sodium, potassium, lithium or ammonium ions across the membrane.
However, a few RFCs do use mildly alkaline electrolytes. For example U.S. Pat. No. 4,485,154 discloses an electrically chargeable, anionically active, reduction oxidation system using a sulphide/polysulfide reaction in one half of the cell and an iodine/iodide, chlorine/chloride or bromine/bromide reaction in the other half of the cell. The overall chemical reaction involved, for example, for the bromine/bromide-sulfide/polysulfide system is shown in Equation 2 below:Br2+S2−2Br−+S  Equation 2
The system may operate at a pH of 6-12; however it is stated that the preferred pH is 7-8. This system does however still utilise a cation exchange membrane to allow cationic charge balancing species to transfer from one half cell to the other, despite the mild alkaline conditions.
Many energy storage and/or energy delivery systems avoid using alkaline electrolytes due to the tendency of many redox couples to precipitate in alkaline media.
As outlined above, one of the major limitations in employing such regenerative fuel cell in industry is the cost of the raw materials, including the ion exchange membrane. Currently, a widely deployed ion exchange membrane in industry is the proton exchange membrane (PEM) based on perfluoro sulfonic acids, for instance Nafion™ produced by DuPont. However as this membrane is costly to produce, much research has taken place to produce cheaper alternatives, including investigation into membranes which exchange anions instead of cations (anion exchange membrane). Tests have been carried out in standard fuel cells which show that these membranes are capable of selectively passing anions, whilst being substantially impermeable to cations.
Varcoe et al, Electrochemistry Communications, 2006, vol 8, p 839-843 describes the synthesis of anion exchange membranes which exhibit a conductivity between 0.010 and 0.035 S cm−1 in the temperature range of 20-80° C., which corresponds to 20-33% of the levels exhibited by NAFION-115 under the same conditions. Yu E. H. et al. Journal of Power Sciences, 2004, vol 137(2), p 248-256 describes a direct methanol alkaline fuel cell which uses an anion exchange membrane. It was found that although the anion exchange membrane used had a higher electric resistance than NAFION membranes, the anion exchange membrane had a lower methanol diffusion coefficient. It is also indicated that expensive precious metal catalysts in the electrodes could be replaced by less expensive non-precious metal catalysts such as nickel or silver, due to methanol oxidation catalyst being less structure sensitive in alkaline media than acidic media.
It is important to realise that regenerative fuel cells are distinct from standard fuel cells. Standard fuel cells consume fuel and can normally only be run in a power delivery mode; they either cannot be run in a storage mode (in which power is stored) or, if they can, they can only do so in a highly inefficient way. Furthermore, reversing the electrochemical reaction in a fuel cell can cause permanent damage to the catalyst. Standard fuel cells are optimised for operating in the energy generating mode only while regenerative fuel cells are optimised for the combined power delivery mode and the energy storage mode. Thus only electrochemical reactions that are readily reversible can be used in a regenerative fuel cell, while in normal fuel cells the reactions need not be reversible and indeed they are usually not. Because of these considerations, regenerative fuel cells will normally use different electrochemical reactions, as compared to standard fuel cells.
Therefore finding two redox couples for use in a regenerative fuel cell that are reversible, soluble at practical concentrations (about 1M or above), and have a suitable potential difference between the standard electrode potentials (Eθ/V) of the couples is a challenging task.
The present invention provides a new class of electrochemical reactions that can be used in regenerative fuel cells and avoids many of the costs of present RFCs and especially uses an alternative to the expensive cation exchange membranes. The new class of electrochemical reactions can allow the use of cheaper reagents in the catholyte and/or anolyte in place of expensive previously used transition metals.